Approaches to increasing the transmission capacity in a wireless transmission system have typically included the use of a multi-valued modulation scheme and the expansion of a wireless band. Unfortunately, the use of a multi-valued scheme involves a short transmission distance due to a low tolerance for fluctuations in a wireless transmission path, and the band expansion may not be allowed due to the absence of specification in related laws. One solution for these problems is the wireless transmission system shown in FIG. 1.
External LAN signal 100 (MAC [Media Access Control] frames compliant with IEEE 802.3) from user network 10 is input to layer 2 switch (L2SW) 50. L2SW 50 divides external LAN signal 100 on a MAC frame basis and outputs the signals as divided LAN signals 110-1, 110-2, . . . , 110-n to wireless transmission devices 60-1, 60-2, . . . , 60-n, respectively. Wireless transmission devices 60-1, 60-2, . . . , 60-n perform wireless modulation for input divided LAN signals 110-1, 110-2, . . . , 110-n and output the modulated signals as wireless signals 201-1, 201-2, . . . , 201-n to opposing wireless transmission devices 70-1, 70-2, . . . , 70-n, respectively. Wireless transmission devices 70-1, 70-2, . . . , 70-n demodulate received wireless signals 201-1, 201-2, . . . , 201-n to extract LAN signals and output the LAN signals as divided LAN signals 510-1, 510-2, . . . , 510-n to L2SW 80. L2SW 80 combines divided LAN signals 510-1, 510-2, . . . , 510-n and outputs the combined signal as external LAN signal 500 to user network 40. Wireless transmission devices 70-1, 70-2, . . . , 70-n perform wireless modulation for input divided LAN signals 510-1, 510-2, . . . , 510-n and output the modulated signals as wireless signals 601-1, 601-2, . . . , 601-n to opposing wireless transmission devices 60-1, 60-2, . . . , 60-n, respectively. In a similar manner, a LAN signal is transmitted in the direction from user network 40 to user network 10. Accordingly, in the wireless transmission system shown in FIG. 1, the transfer capacity can be increased by increasing the number of wireless transmission devices between L2SW 50 and L2SW 80.
L2SW 50 and L2SW 80 include a link aggregation function without LACP (Link Aggregation Control Protocol), as described in Patent Literature 1. Wireless transmission devices 60-1 to 60-n and 70-1 to 70-n include a function of linking down a divided LAN signal port of an opposing wireless transmission device when a link-down state of their own divided LAN signal port is detected, as described in Patent Literatures 2 and 3. Therefore, upon detection of quality degradation of a wireless signal directed either from the local wireless transmission device to the opposing wireless transmission device, or from the opposing wireless transmission device to the local wireless transmission device, the divided LAN signal ports of the local and opposing wireless transmission devices can be linked down. L2SW 50 and L2SW 80 can be notified of the abnormal condition, and the signal transmission can be stopped.
In this manner, combining the L2SWs in Patent Literature 1 and the wireless transmission devices in Patent Literatures 2 and 3 allows increasing the transmission capacity without expanding the wireless band and also allows eliminating the need of complicated LACP processing required in link aggregation.
However, implementing a wireless system with the combination of the inventions described in the above Patent Literatures requires installing the L2SWs apart from the wireless transmission devices, which increases the overall system cost. In addition, the L2SWs as separate devices from the wireless transmission devices pose the problem of cumbersome installation work and maintenance, and the L2SWs as devices different in type from the wireless transmission devices pose the problem of difficulty in providing a common monitoring and controlling network.
Next, combinations different from the combination of the above Patent Literatures will be described with reference to FIGS. 2 and 3.
A wireless transmission system in FIG. 2 connects wireless transmission devices in a star topology, in which L2SW 50 in FIG. 1 corresponds to L2SW 50-1, L2SW 50-2, L2SW 50-n, and L2SW 80 in FIG. 1 corresponds to L2SW 80-1, L2SW 80-2, . . . , L2SW 80-n, and these L2SWs are connected to wireless transmission devices 60-1, 60-2, . . . , 60-n and 70-1, 70-2, . . . , 70-n, respectively. LAN signal 100 from user network 10 is divided into n signals on a MAC frame basis by L2SW 50-1 for wireless transmission device 60-1 in the first stage. One (110-1) of the n signals is output to wireless transmission device 60-1, and the remaining n−1 signals (110-2 to 110-n) are output to n−1 adjacent wireless transmission devices 60-2 to 60-n. LAN signals 510-1 to 510-n transmitted via wireless transmission devices 60-1 to 60-2 and 70-1 to 70-n in n stages are combined by L2SW 80-1 for wireless transmission device 70-1 and are output as external LAN signal 500 to user network 40.
In this manner, in the wireless transmission system in FIG. 2, each of the wireless transmission devices connected to the user networks is connected with its adjacent wireless transmission device in a star topology to provide connection to the user network. As a result, the wireless transmission device aggregates the wireless transmission paths, so that transmission capacity can be increased according to the number of adjacent wireless transmission devices. When an abnormal condition in a wireless transmission path occurs, link down control is performed for an internal LAN signal port, of a relevant wireless transmission device, serving as a connection terminal to the L2SW, to provide notification to the L2SW of the abnormal condition. Since the L2SW does not use the linked-down port for signal transmission according to the link aggregation function, signal transmission using only wireless transmission devices with normal wireless transmission paths is possible.
However, in the wireless transmission system in FIG. 2, the wireless transmission device connected to the user network receives concentrated connections from its adjacent wireless transmission devices. Accordingly, interfaces as many as the adjacent wireless transmission devices are necessary, posing the problem of difficulty in reducing the size and cost of the devices. In addition, the adjacent wireless transmission devices require the L2SW function only to be connected with the wireless transmission device connected to the user network, posing the problem of difficulty in reducing the device cost. Even if the L2SW function is removed from the adjacent wireless transmission devices, it will result in collectively handling the wireless transmission devices of different types, posing the problem of cumbersome installation work and maintenance.
FIG. 3 shows a wireless transmission system with the same internal configuration as the wireless transmission system in FIG. 2 but with different connections between devices. Specifically, the wireless transmission system in FIG. 3 cascades all wireless transmission devices, so that wireless transmission device 60-1 connected to user network 10 is connected with adjacent wireless transmission device 60-2, wireless transmission device 70-1 is connected with adjacent wireless transmission device 70-2, wireless transmission device 60-2 is connected with adjacent wireless transmission device 60-3 (not shown), wireless transmission device 70-2 is connected with adjacent wireless transmission device 70-3 (not shown), and so on.
In the wireless transmission system in FIG. 3, the adjacent wireless transmission devices are cascaded to bundle the wireless transmission paths, so that transmission capacity can be increased according to the number of wireless transmission devices. Once detecting an abnormal condition in a wireless transmission path or in a wireless transmission device, the wireless transmission system in FIG. 3 performs link down control for the internal LAN signal port serving as a connection with the L2SW to notify the L2SW of the abnormal condition. Since the L2SW does not use the linked-down ports for signal transmission according to the link aggregation function, signal transmission can be continued by using only wireless transmission devices with normal wireless transmission paths.
However, in the wireless transmission system in FIG. 3, cost reduction of the wireless transmission devices is difficult because all the wireless transmission devices have the L2SW capable of the link aggregation function. If a simple and inexpensive L2SW without the link aggregation function is applied to the adjacent wireless transmission devices, the result will be that the wireless transmission device of one type connected to the user network and the adjacent wireless transmission devices of another type are lined up. This poses the problem of cumbersome installation work and maintenance.
Further, although the wireless transmission systems in FIGS. 2 and 3 are integrated systems, at the occurrence of an abnormal condition in a wireless transmission path or in a wireless transmission device, the L2SW block of the local wireless transmission device needs to be notified of the abnormal condition in the form of link-down of the internal LAN signal port. This makes fast control difficult compared with directly linking down a LAN port external to the device.